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Internal Mg diffusion (IMD) process can produce MgB 2 superconducting wires with engineering critical current density several times higher than that of traditional powder in tube processed wires, which makes it an attractive and promising method for mass producing practical MgB 2 wires. However, the MgB 2 layer growth stops shortly after the onset of the heat treatment and unreacted B always remained in the MgB 2 layer within MgB 2 wires, negatively affecting the J e performance. Thus it is of great importance to have an in-depth understanding of the mechanism of reaction between Mg rod and B powder forming the MgB 2 layer within IMD wires during heat treatment, and identify the critical factor that controls the formation rate of MgB 2 layer. In present work, the kinetics mechanism of reaction between Mg and B forming MgB 2 layer in the internal Mg diffusion (IMD) processed MgB 2 wires were systemically studied in present work. It was found that the reaction between Mg and B forming MgB 2 layer during the heating treatment is controlled by varied mechanisms. At initial stage, the formation of MgB 2 layer is mainly determined by the rate of chemical reaction between liquid Mg and B powder. As the reaction processes and the thickness of synthesized MgB 2 layer increases, the slow Mg diffusion-limited mechanism gradually becomes dominant at final stage. On the basis, herein Mg rod with a thin Cu coating is proposed to replace normal Mg rod in IMD procedure to accelerate the formation of MgB 2 layer within wires. As a result, Cu coating can change the kinetics mechanism of MgB 2 layer formation and enable the formation of MgB 2 layer to get rid of the restriction of slow Mg diffusion. Complete dense MgB 2 layer without B-rich or unreacted B regions was successfully synthesized within Cu coated IMD wires with larger diameter (1.03 mm) at temperature as low as 600 °C (below Mg melting point). The kinetics mechanism of MgB 2 layer formation determined in our work can provide a valuable guide for optimizing processing parameters of IMD MgB 2 wires. Moreover, the Cu coating technique proposed here opens a promising way to fabricate practical high performance IMD wires at low heating temperature. [ABSTRACT FROM AUTHOR]

Highlights: [•] Glycine doped MgB2 can be made at low temperature by Cu-activated sintering. [•] We obtained excellent J c in glycine-doped MgB2 sintered at 600°C for 15h. [•] Cu and glycine have synergistic effects on improvement of J c. [•] We illustrated the precipitation morphologies of secondary phase MgCu2 and MgO phases. [ABSTRACT FROM AUTHOR]

Self-sintered ex-situ MgB 2 polycrystalline bulks have experienced a two-step sintering process, initially at 900 °C for 0–20 min and then at 650 °C for 1 h. MgB 2 was decomposed to MgB 4 and Mg at 900 °C and composed again at 650 °C from MgB 4 and Mg. The reversible reaction promotes the material migration, and thus eliminates pores and enhances the connectivity between the grains. The critical current density ( J c ) is significantly improved due to both improved grain connectivity and the additive pinning centers such as MgB 4 and new-born boundaries. This two-step sintering process can be a promising method to fabricate high-performance ex-situ MgB 2 bulks and wires. [ABSTRACT FROM AUTHOR]